Original patent literature (EP 0 801 067, WO 9620194) describes a procedure where (1S)-ethyl 1-phenyl-1,2,3,4-tetrahydro-2-isoquinoline carboxylate is reacted with 3-(R)-quinuclidol, which is in about 290% theoretical amount, with sodium hydride, which is in about 156% theoretical amount, in a toluene suspension. The reaction mixture is refluxed, simultaneously distilling off the ethanol being formed, which leaves in the form of an azeotropic mixture with toluene. A reasonable degree of conversion is thereby attained. Nevertheless, the patent states that the yield is as modest as 56%.
The patent mentions also possibility of the synthesis procedure is mentioned where 1-phenyl-1,2,3,4-tetrahydro-2-isoquinoline carbonyl chloride of formula IV is reacted with 3-quinuclidol of formula V in dimethylformamide to obtain 1-azabicyclo[2.2.2.]oct-3-yl 3,4-dihydro-1-phenyl-2(1H)-isoquinoline carboxylate of formula VI.

EP 0 801 067/WO 9620194 also report an arrangement where 3-(R)-quinuclidyl alkyl carbonate is allowed to react with 1-phenyl-1,2,3,4-tetrahydro-2-isoquinoline to give solifenacin.
Patent application WO 2005/087231 describes preparation of solifenacin by reacting (1S)-alkyl-1-phenyl-1,2,3,4-tetrahydro-2-isoquinoline carboxylate (where the alkyl is a lower alkyl, preferably methyl, ethyl, or benzyl) in a toluene-dimethylformamide mixture with 3-(R)-quinuclidol, the reaction being catalyzed by a sodium alkoxide, the alkoxide being so selected that its alkyl is identical with the alkyl of the (1S)-alkyl-1-phenyl-1,2,3,4-tetrahydro-2-isoquinoline carboxylate (hence, sodium methoxide, ethoxide, or benzyloxide).
We have found empirically that all the procedures published so far suffer from drawbacks. The procedure in EP 0 801 067 does not afford the product in a reasonably high yield and, in addition, uses a comparatively high excess of optically pure 3-(R)-quinuclidol which is rather costly.
The other route—reaction of 1-phenyl-1,2,3,4-tetrahydro-2-isoquinoline carbonyl chloride of formula IV with 3-quinuclidol of formula V in dimethylformamide, resulting in 1-azabicyclo[2.2.2.]oct-3-yl 3,4-dihydro-1-phenyl-2(1H)-isoquinoline carboxylate of formula VI—is mentioned in patent literature (EP 0 801 067) but no examples of the procedure are given. We have tested the procedure practically and found that, although the reaction between the two components, the carbamoyl chloride and quinuclidol, proceeds at a reasonably high rate without any additional catalysis (monitored by HPLC), an intermediate seems to be formed, associated with acylation at the nitrogen giving a quaternary ammonium salt. This salt is hydrolyzed by water which is added within the product isolation step, and the acid formed is decarboxylated immediately. The forming 1-phenyl-1,2,3,4-tetrahydroisoquinoline of formula VIII
reacts immediately with the residue of the acylation agent (intermediate) giving, in a nearly quantitative yield, an urea according to the following Scheme 1.

Furthermore, we have found experimentally that catalysis using an unbranched sodium alkoxide, as described in WO 2005/087231, is inconvenient because the unbranched alkoxide acts not only as a base but also as a nucleophilic agent, whereupon an impurity is formed—solifenacin substituted by an alkyl group in position 2 at the quinuclidine backbone.